Extremely refractory ceramic materials, by their very nature, present a number of very difficult problems from a commercial processing standpoint. Their refractoriness customarily demands the use of ultra-high firing temperatures both to synthesize and to fabricate such materials into useful shapes. Those high temperatures practically guarantee that any laboratory process devised for synthesizing and shaping articles from very highly refractory materials will either be too uneconomical to justify scale-up to commercial production, or will require operating conditions exceeding the capabilities of standard manufacturing equipment.
An illustration of that situation is found in the mineral pollucite, a cesium-containing feldspathoid having the composition Cs.sub.2 O-Al.sub.2 O.sub.3 -4SiO.sub.2 (abbreviated CAS.sub.4). Pollucite is the most refractory silicate known, exhibiting a melting point above 1900.degree. C. In addition to its high refractoriness, pollucite also demonstrates high temperature stability, low elastic modulus, a relatively low linear coefficient of thermal expansion, viz., about 20-30.times.10.sup.-7 /.degree.C. over the temperature range of 0.degree.-1000.degree. C., thereby rendering it suitable for applications incurring thermal shock, and displays good resistance to attack by water and alkaline solutions.
The extreme refractoriness which makes pollucite such a desirable material also, unfortunately, makes it extraordinarily difficult to synthesize via traditional approaches. To illustrate:
U.S. Pat. No. 3,723,140 (Beall et al.) discloses the preparation of glass-ceramic bodies having crystal contents greater than 50% and, commonly, greater than 80% by volume, wherein pollucite constitutes the predominant crystal phase. The precursor glasses therefor required melting temperatures of 1850.degree.-2000.degree. C. in rhodium crucibles. Such processing conditions would be extremely difficult to justify for any large scale commercial operation.
In like manner, the preparation of pollucite via the sintering together of its individual constituent oxides has proven to be very difficult, again demanding extremely high firing temperatures.
Accordingly, the primary objective of the instant invention was to devise a method for synthesizing pollucite utilizing temperatures compatible with standard manufacturing equipment.
Hence, whereas higher temperatures can be employed, a particular objective of the present invention was to devise a method for preparing bodies of pollucite utilizing temperatures not exceeding 1650.degree. C.